Criopreservación de ovocitos humanos. Vitrificación vs. Congelación

Human oocyte cryopreservation has become an essential technique in Assisted Reproduction laboratories. The clinical applications of this technology should ensure optimal survival of the embryos and oocytes that are stored and subsequently thawed for transfer. The aim of this review is to compare the slow cooling procedures with vitrification. The two most important parameters which determine the success of the cryopreservation procedure are the way that cells reach the osmotic equilibrium and the cooling rate. Slow-rate freezing protocols result in ice crystal formation during cooling and warming. Vitrification combines ultra-rapid cooling rates with high cryoprotectant concentrations that completely eliminate ice formation during cooling and warming procedures. The most important drawbacks of this technique are the need to achieve more rapid cooling rates and to determine the maximum cryoprotectant non toxic concentration for the cell. Although survival rate after thawing depends on cell type, chronobiology and embryo transfer quality, the success of the vitrification technique can be observed. The number of pregnancies and healthy births after cryopreserved oocyte transfer shows that vitrification is a simple, fast and safer procedure and that it could be the technique of choice for clinical cryopreservation.